This invention relates to clutch drag controls for a hoist drum. Such controls are utilized to rotate the drum at desired speeds and in forward and reverse direction to raise and lower a load in a material handling device, such as a large vehicular crane or the like. Many hoists in such devices include a pair of drums to add flexibility of operation.
Heretofore, the controls for such drums have at times included a drum clutch control pressure graduating valve which was connected between a source of pressurized air and a shuttle valve, with the shuttle valve being connected in a line through a rotary union or joint at the drum shaft end to the clutch cylinder for the drum drive. A quick release valve was disposed closely adjacent the clutch cylinder for releasing pressure in the clutch line when desired.
In such prior constructions, an adjustable drag regulator and air supply poppet valve were connected between the source of pressurized air and the shuttle valve. In a dual drum system, when it was desired to apply a lower than full drive pressure to one drum drive clutch cylinder so that the drum could be utilized in a slow speed dragging operation compared to the other drum, the one drum clutch control pressure graduating valve was placed in neutral and the air supply poppet valve was actuated to supply air at reduced pressure to the shuttle valve.
At times of drag operation, it has often been desirable to momentarily return the one hoist drum to full speed, for example to keep the drum cable traveling at the same speed as the cable on a companion full-speed drum. This has previously been accomplished through circuitry wherein momentary actuation of the drum clutch control pressure graduating valve to an out-of-neutral mode has replaced the low pressure in the line downstream of the graduating valve with high pressure.
Problems have arisen with the above-described controls.
After the momentary increase to full drum speed described above, the system should promptly return the drum to slow dragging speed. Previously, it was noted that even though the controls were returned to drag mode, the drum would continue to rotate at fast speed for a time. The operator, following the settings of the controls in front of him, would think the drum was dragging, when in reality it was not. In some instances, the hook block or other attachment at the outer end of the hoist cable would sometimes quickly raise all the way up to the boom tip before the situation was discovered, requiring emergency fast action to stop the drum from rotating further.
In addition, and even with no drum dragging operation, it was noted that when the drum clutch control pressure graduating valve was shifted from a drive mode to a neutral mode, indicating at the console controls that the drum clutch was released by its cylinder, the drum would keep right on rotating so that cable movement undesirably continued for a number of feet before coming to a stop. Insertion of a quick release valve in the line closely adjacent the clutch cylinder to drain the said line was found not be be truly effective. Attempts over many years to solve the problem have not been successful.